With the growing capacity demand for optical fiber communications, wavelength add/drop multiplexers ("WADM") are essential components in any optical network. In particular WADMs are critical components in wavelength division-multiplexed ("WDM") regional-access ring or bus networks to provide access to local customers.
Current technology utilizes configurable wavelength 2.times.2 switches inserted in wavelength paths. FIG. 1, which is prior art, depicts an example of a conventional WADM architecture. The conventional WADM includes input port 140, demultiplexer 110, multiplexer 120, output port 130 and a plurality of 2.times.2 switches 105(1)-105(M). A WDM signal including a plurality of multiplexed signals .lambda..sub.1 -.lambda..sub.M is received at input port 140 and transmitted to demultiplexer 110. Wavelengths .lambda..sub.1 -.lambda..sub.M received via local access ports (not shown) may be added via respective switches 105(1)-105(M). Conversely, wavelengths .lambda..sub.1 -.lambda..sub.M from the demultiplexed signal may be dropped via switches 105(1)-105(M) to local access ports (not shown). A particular wavelength .lambda. is dropped to and added from the local port if the respective 2.times.2 switch (105) is in a cross-state, while it is sent directly to output port 130 when the switch is in a through state. 2.times.2 switches 105 may be of a discrete or integrated form.
Ring networks have become very popular in the carrier world as well as in enterprise networks. A ring is the simplest topology that is two-connected, i.e., provides two separate paths between any pair of nodes. This allows a ring network to be resilient to failures. These rings are called self-healing because they incorporate protection mechanisms that detect failures and reroute traffic away from failed links and nodes onto other routes rapidly. A unidirectional ring carries working traffic only in one direction of the ring (e.g., clockwise).
FIG. 2a, which is prior art, depicts the topology of a unidirectional ring network. A unidirectional ring network carries working traffic in only one direction of the ring (e.g., clockwise), along service fiber 230. WADMs 210a-210d provide functionality for dropping and adding wavelengths via local access ports 220a-220d respectively. For example, working traffic from WADM 210a to 210b is carried clockwise along the ring and working traffic from WADM 210b to 210a is also carried clockwise on a different set of links in the ring. Protection fiber 240 provides a backup route in the case of a fiber cut or equipment malfunction in the working fiber 230. Traffic from WADM 210a to WADM 210b is sent simultaneously on working fiber 230 in the clockwise direction and protection fiber 240 in the counter-clockwise direction.
FIG. 2b, which is prior art, depicts the topology of a bi-directional two-fiber ring network. Note that both fiber routes 230a and 230b in FIG. 2b carry a non-overlapping sub-set of wavelengths (e.g., even and odd number wavelengths). Thus, both fiber routes 230a and 230b are working/protection fiber since one direction can function as the protection route for the other direction (because the wavelengths are non-overlapping). For example, in an even/odd arrangement, signals in the protection routes would be even number wavelengths in odd number wavelength fiber routes and odd number wavelengths in even number wavelength fiber routes.
Typically, WADMs require additional functionality to enable loop-back for maintenance or to switch the signal to a restoration path in the case of a fiber cut or equipment malfunction. FIG. 3, which is prior art, depicts typical connectivity requirements for a WADM in a uni-directional ring network. WADM 210 must be able to switch signals from WS.sub.IN (west service input) 230a to WP.sub.OUT 240b (west protection output) for loop-back maintenance. Also, if a failure or fiber cut occurs on the east side of WADM 210, wavelengths from local access ports 220 must be switched to WP.sub.OUT 240b for restoring the network traffic. Likewise WADM 210 must switch signals arriving from WS.sub.IN 230a originally destined for ES.sub.OUT 230b to WP.sub.OUT 240b.
Although the functions required as shown in FIG. 3 may be achieved by a 3.times.3 cross-bar matrix or three 1.times.3 switches for each wavelength path, the utilization of switch points is inefficient. This results in an increase of the complexity of the electronic controls, size and cost of the WADM device.